On the origin of the electronic anisotropy in iron pnicitde superconductors

Abstract

We use polarization-resolved Raman spectroscopy to study the anisotropy of the electronic characteristics of the iron-pnictide parent compounds $A$Fe${}_2$As${}_2$ ($A$~=~Eu, Sr). We demonstrate that above the structural phase transition at Ts the dynamical anisotropic properties of the 122 compounds are governed by the emergence of $xy$-symmetry critical collective mode foretelling a condensation into a state with spontaneously broken four-fold symmetry at a temperature $T^{\ast }$. However, the mode's critical slowing down is intervened by a structural transition at Ts, about 80~K above $T^{\ast }$, resulting in an anisotropic density wave state.